We have generated constructs for dbl expression by taking advantage of the availability of expression vectors carrying various cell type-specific transcriptional control elements. These constructs were used to generate several lines of transgenic mice carrying the in oncogene. Independently of the promoter used, transgenic mice exhibited a dominant bilateral dysplasia of the lens similar to that induced in transgenic mice by expression of the in genomic clone. We have identified a region of significant similarity shared among the predicted translational products of the human dbl proto-oncogene, the Saccharomyces cerevisiae cell division control gene, CDC24, and the human breakpoint cluster gene, bcr. We tried to investigate the functional meaning of this homology and to use CDC24 as a source of information for the dbl function. Deletion of amino acid stretches of various lengths, as well as conservative or radical substitutions within this region of homology, led to drastic reduction of the transforming activity of the dbl gene.